A UMTS communication system comprises at least a base station (BS) referred to as Node B and a mobile phone terminal referred to as user equipment (UE) throughout the document. Both the Node B and the UE comprise a transmitter and a receiver. The Node B sends signals by means of its transmitter to the UE receiver in the downlink. The UE sends signals by means of its transmitter to the receiver of the Node B in the uplink. In uplink and downlink communication both the UE and the Node B map the data to be transmitted into physical channels.
The UMTS standard provides two physical channels for downlink communication, a first channel denoted “Common Pilot Channel” (CPICH) consisting of a continuous sequence of known pilot symbols, and a second physical channel denoted “Dedicated Physical Channel” (DPCH). The DPCH consists of a first logical sub-channel denoted “Dedicated Physical Data Channel” (DPDCH) which carries the information data symbols intended for the user concerned, time-multiplexed with a second logical sub-channel denoted “Dedicated Physical Control Channel” (DPCCH) which carries known user-dedicated pilot symbols.
FIG. 2 shows slot structures (each slot consisting of 2560 chips) provided in fifteen slots S0 to S14 of the above two different pilot sequence bearers in the UMTS downlink. The CPICH channel continuously transmits common pilot symbols CP, while the DPCH channel intermittently transmits dedicated pilot symbols DP in the DPCCH sub-carrier.
It is noted that the term “physical data channel” such as DPCH and CPICH, denotes a channel which carries the data, while in the following the term “transmission channel” is intended to cover also signal processing elements or functions, such as pulse shaping and pulse shape matched filter (or antialiasing filter), which distort the data.
The UE receiver needs to estimate the transmission channel parameters in order to demodulate, decode, and recover the information data contained in the DPDCH channel. In the absence of transmit beamforming the transmission channel parameters associated with the CPICH channel and the DPCH channel are fairly the same. The continuous provision of known pilot symbols of the CPICH physical channel can be easily used at the UE receiver to estimate the related transmission channel, which is the same as the transmission channel associated with the DPCH channel, and then use this estimate for DPDCH information data recovery. In general, known channel estimation techniques proposed for UMTS receivers were generally based on either the common pilot sequence of the CPICH channel or on the dedicated pilot sequence provided in the DPCH channel.
Transmit beamforming can be employed at the Node B during downlink communication to improve the reception at the UE of the user information data mapped into the physical channel DPDCH. To perform transmit beamforming, the Node B needs to be equipped with multiple antennas. In this way it is possible to provide optimal spatial filtering by shaping beams in the directions of the user concerned while setting nulls in other user directions to mitigate the interference to other users. While transmit beamforming is to be employed for the transmission of the physical channel DPCH, it is not suited for the physical channel CPICH transmission that is to be uniformly broadcasted in time and space (i.e. in all directions) to all users.
The channel estimation approach relying on the DPCCH channel has limited accuracy due to the reduced number of dedicated pilot symbols DP per slot. Furthermore, the lack of dedicated pilot symbols DP during the DPDCH period prevents tracking fast channel variations. The classical channel estimation approach based on the common pilot symbols CP of the CPICH channel can better adapt to fast fading conditions. However, it cannot be used in the presence of beamforming.
There have already been some proposals for path-wise dedicated channel estimation, which make use of both dedicated and common pilots. However, they assume that they have perfect a priori knowledge of the path delays and the channel associated with the DPCH is identical to the one associated with the CPICH, which is not the case when there is beamforming.
Additionally, the US 20040102203-A1 discloses channel estimation at a UE in the presence of transmit beamforming, wherein more than one pilot source is exploited. The proposed channel estimation is based on a pathwise (sparse multipath) channel model and correlators are used to estimate the multipath parameters independently at a first step and at a second combining step. Channel estimates obtained from several pilot sequences are explicitly combined. The method is suitable for Rake type receivers.